5.15 is still a pretty old kernel. Try an up-to-date Fedora Live USB. It would be weird if Zen5 EDAC wasn’t supported (one would think…).
The on-die ECC wouldn’t affect the bus width though. On-die ECC memory should still show as 64 bit?
The reason dmidecode bit width doesn’t tell the whole story, AFAIK, is that it reports just what the UEFI read from the DIMM SPD. And the fact that the DIMMs themselves are 72 bit wide doesn’t mean ECC error correction is enabled and working.
@IrY100Fan: To truly know that ECC is working you need to trigger errors and see that they are corrected. For this you need working error reporting (otherwise how would you know an error occured in the first place?). If you try this, make sure Platform First Error Handling is disabled in UEFI since that setting masks reporting to the OS. See more in the linked thread…